Technical stuff - Not stocks - Inhibitor Switch

This is some technical info about cars with  auto transmission gears . The story is directly related to something happened to me during last few days. Non of the four garages visited by me diagnosed the troubles of my car and I almost lost few laks of money due to wrongly  diagnosing  and actions done by mechanics. That is why I give this information to others. One of the garages is said to be  operated by a BSc qualified Engineer. (not Mr Alavaththa)

The entire episode is related to a gadget called 'Inhibitor switch' and I found no one in those repair places knew correctly about it.
I write this because the know-how  may help someone  someday.

This is a long story.
If you are not interested in 'gajematics' do not read it.

In auto geared (automatic transmission) cars you can see the gear position in the dash board. When you change the gear lever to P, N, D , R or 1/2 positions it is indicated by lighting the correspondent bulb in the dash board.

Power suply to the  starting motor is also connected through the gear lever .  Power to the starting motor is only connected when you place the lever in P or N positions. Hence in other positions you cannot start the car.

My problem started with errornous indications in the dash board.   When I changed to D to run the vehicle it shows N although the car runs  at D.  When the troubles continued I couldn't start the car at P but sometimes it get connected at R or even D.

At the same time when I  change the  gears the engine shake like doing it half  clutch in manual cars.
When I changed from N to D to start running it made  'dug' sound with big shake in the engine and it jump forward with unusual high power.

At this stage I consulted  garage people, two places.


One fellow had  a test run and diagnosed 'gear cable malfunctioning' or kebal eka edila. So you have to replace the cable to get the correct indication in the dash board.  
 For the shaking of the engine , he suspected broken engine mounts .
I went to a reliable  cable repair place near Subadrarama temple closer to Gamsabha junction, Nugegoda . They correctly said no error in the cable and didn't charge me a cent and asked me to check the 'gear changing switch'. I did not know where is this switch initially. Later I found that is called the Inhibitor Switch and located externally at the top of the gearbox  .

I went to another garage I know. They inspected and gave reasons for the shaking of the engine when changing from N to D.
Diagniosed: Engine mount failure. Some mounts broken or loosened. Replacement cost 6500 + 7500 x 2 = approx 30,000.00  He also suggested changing of the gear cable.
He almost started the job but fortunately, I had an important work and took the car away.

At this stage I could start the car at D or R positions and not P or N. Apart from the big shake  at changing  N to D or from N to R, I had no trouble when running. I was instructed by the both places if I can start at D and do  the running there will be no broblem at all, irrespective of what indicated in the dash board.  Sir, that is just to show you the lever position and nothing else, they said.


Within one week another major trouble started.  I felt the gearbox is working only at 1 and 2 gears.  Normally it should  change  1 to 2 at pickup and then to 3 to top levels with the increasing speed, automatically.

Now  it changes only once, from lowest to the next .  I could not run beyond 35kmph and RPM was much higher than usual.

Again I took the car  to the garages . Now the diagnosis was gearbox failure.
As per the experts, all the troubles I had before were signs of this final outcome.  Some wheels or parts of the automatic gearbox has damaged and you can't run the car with it. Only Low gear wheels are not broken yet. Repairing is not advised and what they do normally is fixing a new gearbox.  Cost of a used  imported gearbox is around 135-150,000.00

No other option now.   I have to find  two laks.

The total job:

Engine mounts : 30,000.00
Gearbox  150,000.00
Inhibitor switch 6000.00
labour cost 25,000.00 approx.

Total 211,000.00



 I was not satisfied at all about these garage people in the whole drama. I asked them the functioning of the Inhibitor swith in deep buy they couldn't give me satisfactory answers.
As per them the switsh is to track your lever position and indicate it in the dash board and cut the ignition power when you are not at Parked or at Nutral . Nothing else.

I felt suspicious. All started with wrong indication of gear positions at the dash board . That means the inhibitor switch is broken.
After that the engine and the gearbox started malfunctioning.

Hence, without letting others to remove the broken gearbox  I decided to check the inhibitor switch my self.

I removed the battery and its underneath panel to open inhibitor switch area. Normally it is placed under battery at the top of gearbox. (see image)



There was a lever to connect it to the gear cable. The cable is connected to the hand stick inside the car. Inhibitor switch rotates when you change the gears and connect particular wires to send current to dash board and other places.

I removed and dismantled the switch and found the plastic 'hand' inside  was broken.
Hence bought a new one paying 5,500/- with 15% discount.

I connected the new one to the wires and then fixed it to the gearbox  .
The correct positioning of gearbox and the switch can be achieved using indicator holes. They are aligned at N position.  Then you have to place the gear lever (inside the car) at N position and connect the cable to the lever at top of the switch . In this way you place N inside the gearbox, N at the switch and N at the gear lever fixed to each other.

Connecting current wires to the switch was the most dificult job. It has 9 connectors.

After fixing everything I started the car.

Changed to D. Nothing happened. For a second  I thought even first gear is not functioning.
Within second car started to move. I accelerated and had a run .
Amasingly everything became normal . Gear changing is as good as or even better than a new car,  done at correct speeds very smoothly . No shakes. No wrong indicator lights. RPM much lover .  
No mount problems. Gearbox is perfect.

All the trouble was due to disconnected Inhibitor switch.
Not only to the dashboard,   it connects power to reverse light and   connects some lines to the car computer.  
Actually gear controlling computer 'sees' the gear position using the signal from Inhibitor switch.  Without the correct signal the controller cannot guide the gearbox to change the gears smoothly.
That makes it behave badly.

If anybody get this type of problems better to check and replace this switch  before going for big work or replacements.

Later I realized there is a designers laziness involved here.
Instead of getting feed back from the gearbox   to 'show'  the gear position to the controller, the designers has done it in a very bad way.

They fixed a switch  in between the the hand  gear lever and  gearbox and monitor the gear cable  to determine what inside the gearbox. If the gear cable elongated or some connectors in the switch weathered or  broken its signals become not tallying with actual position.  As the controller is blind to see the actual gear box  and they made it to 'think' the signal from the switch is from the gearbox it guides the gearbox wrong way.

My total cost was 5,500/- +travelling.
By doing it myself I saved approx 200,000.00.

Seems a new invention by Mr. Chinwi.   It would be useful for the relevant people.

Thanks Chinwi and I have had somewhat similar issues and I will get this switch checked up first thing on Monday morning.
Thanks again.

Great lesson learnt. Tks chinwi

excellent post.. A good overall view as well. Thanks Chinwi

Thanks. Its good that you found a design issue also here. ?Why don't you write to the parent company about it. They might pay all what lost + some more for the find.

Great info thanks for caring to share.

When I have about 150ml solution of achohol mixed with 3 parts of tonic with some ice and a slice of lime, my inhibitor switch goes of automatically and I start behaving in strange but nice ways, don't even ask about the mounts.

Ethical Trader wrote:Thanks. Its good that you found a design issue also here. ?Why don't you write to the parent company about it. They might pay all what lost + some more for the find.

Thanks. Now we know everybody do it same way. Even modern BMW, Volvo, Mercedes models share same mechanism.
Actually this is reliable as long as functioning normally. The problem was our people were not aware of its functionality and possible errors.

This post was picked up on Autolanka forum within minutes That probably would have been the perfect place to get a solution as well.

Thanks,Engine mounts you dont need to replace with new ones.you can change the rubber core.

Thanks Chinwi, very good information.

The following video is about a whole new way of looking at the drive mode indications.

https://www.youtube.com/watch?v=JMJhOSdxQVg

Great stuff Chinwi,+Rep
sometimes I wonder about your talents,you have a great knowledge in many subjects